Patients suffering from a variety of medical conditions require supplementary respiratory support. Depending on the nature and severity of the condition, this respiratory support can range from the provision of an elevated oxygen concentration cloud to the vicinity of the nose and mouth, to forced ventilation of the lungs by intubation of the trachea. In general, a supply of pressurized air or therapeutic gas is provided by a tube or conduit to a delivery apparatus designed to conform to particular body structure.
One style of delivery apparatus is a mask which provides the gas to the nasal area of the patient. Nasal masks are often employed in the treatment of sleep apnea syndrome, characterized by intermittent upper airway obstruction during sleep. Due to the resulting blood oxygen desaturation and frequent arousals from sleep, persons suffering from this condition are often unable to achieve deep sleep for extended periods and are chronically tired and physically compromised. In severe cases, cardio-pulmonary complications can arise or death during sleep may result.
Since nasal masks are often worn by persons in unmonitored environments for extended periods, such as in the home during sleep, the nasal mask should be comfortable to wear and conform well to the nasal area thereby defining a sealed chamber. If the mask is deemed too bulky, too heavy, or to fit poorly, the patient will either not wear the mask, wear the mask improperly, or only wear the mask occasionally when the discomfort associated with the respiratory condition exceeds the discomfort of wearing the mask.
One problem associated with nasal masks relates to the conformance of the mask to the nasal area, which is complexly contoured and differs from patient to patient. Customized masks manufactured to suit particular patients tend to be costly; therefore, masks for general use are made in several generic sizes, each size designed to accommodate a range of patients. If the mask does not form a good seal around the patient's nose, leakage can occur, reducing the effectiveness of the treatment. When poorly fitting masks are used with variably regulated air supply systems responsive to patient breathing, such as those developed for treating sleep apnea, mask leakage can induce improper system response which may exacerbate the patient's condition. Regulated air supply systems for treatment of sleep apnea are disclosed in Patent Cooperation Treaty international application number PCT/US93/05095 published on Dec. 9, 1993 as international publication number WO 93/24169; U.S. patent application Ser. No. 08/184,976 filed Jan. 24, 1994; U.S. patent application Ser. No. 08/331,030 filed Oct. 27, 1994 now U.S. Pat. No. 5,645,054; U.S. Pat. No. 5,199,424; U.S. Pat. No. 5,245,995; and U.S. Pat. No. 5,522,382, the disclosures of all of which are herein incorporated by reference.
One method of reducing leakage is to provide a compliant sealing flange or surface around a perimeter of the mask in combination with a strap to bias the mask into sealing engagement with the nasal contour of the patient. Typically, the greater the retention force applied by the strap, the better the seal; however, both the strap and the mask can cause excessive pressure on delicate areas resulting in irritation and patient discomfort.
The retention force required to prevent leakage is also a function of forces and torques induced in the mask. For example, the weight of the conduit supplying air or gas to the mask tends to pull the mask downward, away from the patient's nasal area, when the patient is sitting. Additionally, any movement of the head from side-to-side or up and down can cause lifting of an edge or sliding of the mask and strap. The more rapid the movement, the more pronounced the effect. Mask slippage and displacement are exacerbated in masks employing large diameter, heavy, or stiff tubes which deliver relatively large volumetric flow rates of air, such as those employed in sleep apnea treatment systems. Masks have been developed which employ a swivel attachment between the conduit and the shell. While these attachments can alleviate some movement induced forces, they are effective only within the planar degree of freedom of the swivel. These attachments are ineffective in reducing forces in other planes and do not address offsetting the weight of the conduit.
For nasal masks used by patients when sleeping, the strap and seal arrangement should also accommodate unconscious or reflexive head and body movements. The discomfort associated with masks which apply too much pressure to the head, neck, or nasal area discourage use of the mask during sleep when it is most needed. As a result, treatment is compromised and the patient is ill served by the apparatus.
Accordingly, there exists a need to overcome the limitations of known designs by providing an improved nasal mask which provides a consistent, reliable nasal area seal while being comfortable to wear. Other desirable features would include ease of manufacture and low cost.